The present invention relates to a coupled cavity type traveling wave tube, and more particularly to a structure of a non-reflective termination for a slow-wave circuit in such tube.
Generally, a coupled cavity type traveling wave tube is composed of an electron gun for projecting and forming an electron beam. An electromagnetic wave interacts with the electron beam and is amplified in a coupled cavity type slow-wave circuit. A non-reflective termination means divides the slow-wave circuit with respect to a high frequency wave for the purpose of preventing oscillation. A collector captures electrons which have finished their interaction with the electromagnetic wave and dissipates heat energy. A focusing device is used for maintaining the diameter of the electron beam at a certain fixed size along the slow-wave circuit.
An electromagnet or a periodic permanent magnet (PPM) has been used as a focusing device in a coupled cavity type traveling wave tube. However, the focusing device employing the PPM is more often utilized because it is compact, light in weight and does not need a power supply for excitation as would an electromagnet. However, the PPM used as a focusing device in a coupled cavity type traveling wave tube has a number of shortcomings. The greatest shortcoming is that the magnetic field strength necessary for focusing an electron beam to a certain fixed diameter is essentially determined by the inner diameter of pole pieces in the PPM. When independently forming a PPM for a coupled cavity type slow-wave circuit, the inner diameter of the pole pieces in the PPM is limited by the diameter of the cavities in the coupled cavity type slow-wave circuit. Hence it is difficult to obtain the magnetic field strength necessary for focusing an electron beam.
It is particularly difficult to obtain the necessary magnetic field strength at an input waveguide section for guiding an input electromagnetic wave to the coupled cavity type slow-wave circuit; at an output waveguide section for externally deriving an electromagnetic wave from the coupled cavity type slow-wave circuit; and at a non-reflective termination section for severing the coupled cavity type slow-wave circuit with respect to a high frequency wave. This is because one or or more of the magnets of the PPM must be omitted. The problems at the input and output waveguide section may be avoided by the technique disclosed, for example, in the copending U.S. patent application Ser. No. 137,799 filed on Apr. 7, 1980. However, the problem at the non-reflective termination section heretofore remained unsolved.
One approach in the prior art for avoiding the omission of a magnet of a PPM at the non-reflective termination section, that is, for avoiding imperfection of a PPM at the non-reflective termination section, is disclosed in Siemens Review, Vol. 34, No. 2, p-p. 60-68. Particularly as shown at FIG. 1, two wedge-shaped lossy ceramic bodies are disposed to form a non-reflective termination for a forwardly traveling wave. Two similar bodies are disposed to form a non-reflective termination for a backwardly traveling wave. These wedge-shaped lossy ceramic bodies are disposed so as to extend over a plurality of cavities to improve matching characteristics. However, this approach has at least one shortcoming in that the lossy ceramic bodies are placed within resonant cavities. As a result the impedance of the circuit varies largely and hence it is difficult to achieve proper matching.
According to another approach disclosed in U.S. Pat. No. Re. 25,329, especially in FIG. 2 thereof, two lossy bodies have a cross-sectional configuration which is the same as the shape of the coupling hole in the slow-wave circuit, disposed in one cavity. One is provided for a forwardly traveling wave and the other for a backwardly traveling wave. In this approach, it is difficult to obtain good matching characteristics since the impedance of the slow-wave circuit changes largely abruptly due to the lossy bodies.